hey WilSurf,
I have not personally installed or used these units in the link below, but have looked at a similar setup as you are describing
I know you can get amps that will accommodate multiple inputs to different speakers (ie kids watching TV on one set, adults listening to music out side). All depends on how much you want to spend.
Check out these units if you have not already
http://www.sonos.com/In the end, I decided on a much cheaper solution.
Streaming music from my phone / tablet device to a bluetooth portable speaker (the quality of these speakers are fantastic now, even battery operated ones). Best thing is this solution also travels with us on our camping trips.
I thought I was pretty up to date on technology, but only last month a mate of mine showed me pandora. WOW, how cool. now I stream music of my choice to my Bluetooth speakers for free without pirating or ripping mp3's.
implementing something like this into an existing stereo really depends on what connectivity features your stereo has.
sorry, probably not really any help, but hopefully may give you something to think about re other solutions
Justin